1. Field of the Invention
The invention relates to a transmission system comprising a transmitter, which transmitter includes a speech coder that has a memory arrangement for storing excitation signals, a filter arrangement for filtering the excitation signals, and selection means for comparing a signal derived from the speech signal with the output signal of the filter and based on such comparison selecting the optimum excitation signal. The transmitter further includes a detector for detecting speech pauses and turning off at least parts of the speech coder when a speech pause is detected, and means for transmitting the optimum excitation signal to a receiver. The receiver includes a speech decoder for recovering the optimum excitation signal and the speech signal.
2. Description of the Related Art
Such a method of coded speech transmission is widely known, for example from the text book "Advances in Speech Coding" by Bishnu S. Atal, Vladimir Cuperman, and Allen Gersho, 1991, Klower Acad. Pub., more specifically, pages 69 to 79. This method is especially used in mobile radio for transmitting speech signals between a mobile station and a fixed station. The mobile station is generally battery-operated and, as the transmitter consumes the most power, it and the associated components are turned off during speech pauses to save energy and extend the useful life of the batteries. Due to the highly complex structure of the speech coder, however, the coder requires considerable power. This is especially because all the memory locations of the memory arrangement are to be addressed during each speech frame and all the excitation signals, also termed excitation vectors; are to be filtered to find the optimum excitation vector i.e., the one which provides, for example, the least energy in the difference signal produced by the difference forming stage.
WO 93/13516 describes an arrangement for performing the aforesaid method but without giving details for the speech coder. Therein the speech coder is turned off during speech pauses and only few parameters, i.e. LPC coefficients and autocorrelation coefficients, are further produced, from which parameters the detector detects the speech pauses and also from which parameters information is derived for background noise to be transmitted. It may be assumed that the filter arrangement in the speech coder is then also turned off, because the output signals thereof are not directly necessary during speech pauses. When, however, the speech signal recommences, the filter needs to have a certain time to build up to full intensity after being turned on, so that non-optimum parameters for the transmission of the speech signals occur during a transition period.